A method of obtaining a piezoelectric single crystals with polydomain structure for precise positioning devices

 

The invention relates to the field of obtaining single crystals of ferroelectric domain structure formed and can be used when creating and working appliances precise positioning, in particular probe microscopes, as well as during alignment optical systems. The essence of the invention: production method of a piezoelectric single crystals with polydomain structure for devices accurate positioning of the first form of the ferroelectric single crystals, in which formation is possible only 180data domain boundaries, the workpiece. In this workpiece, at least two faces parallel to each other. The perpendiculars to these parallel edges do not coincide with the direction of the axis of spontaneous polarization. Then move the workpiece in thermal field in the furnace zone temperature above the Curie temperature in an area with a temperature below the Curie temperature. Simultaneously to the parallel faces of the workpiece is applied periodically varying alternating electric field. After cooling the entire volume of the workpiece below the Curie temperature it formed the specified domain structure. The sizes of the domains in the structure are controlled by the speed of movement of merchantable is and plates, two faces which are parallel to the domain boundary and contain an equal number of domains of opposite polarity. The technical result is achieved by enhancing the capacity of the single crystal due to the increase in the area of domain boundaries and volume domains, the possibility of orientation of the polarization vector of the domain at any given to domain wall angle, to obtain the domain structures as strictly regular in the whole volume, and any law change. 7 C.p. f-crystals, 8 ill.

The invention relates to the field of obtaining single crystals of ferroelectric domain structure formed and can be used when creating and working appliances precise positioning, in particular probe microscopes, as well as during alignment optical systems.

Known method of monodominance ferroelectric crystal such as lithium niobate, comprising annealing the crystal at a temperature above the Curie temperature and the influence of an electric field of a certain polarity on the crystal. Further, when the temperature is below the Curie temperature on the crystal again affected by the electric field of the opposite polarity. As a result of processing in the single crystal formerstate of this method is the inability to use the obtained single crystals as pestujhely devices precise positioning.

Also known is a method of obtaining periodic domains by affecting the crystal high-voltage pulses applied to the electrodes located on opposite faces of the crystal, and applied lithographic method. This method has good repeatability domains (N. Ito, C. Takyu, N. Inaba. Fabrication of periodic domain grating in LiNbO3by electron beam writing for application of nonlinear optical processes. // Electronics Letters, 1991, V. 27, P. 1221-1222).

The disadvantage of this method is the inability to polarize the crystal thickness more than 0.2-0.5 mm, to obtain only a small square with domains up to 1 cm2and frequent electrical breakdowns and mechanical fracture (cracking).

Closest to the invention is grown the way to obtain the domain structures when exposed to the crystal temperature gradient in the process of its growth, the so-called growth of lamellar domain structure (N. F. Evlanov, I. I. Naumov, I. O. Chaplin and other Periodic domain structure in crystals LiNb3:Y grown by Czochralski method. // Solid state physics, 2000, T. 42, vol.9, S. 1678-1681).

The disadvantage of this method is the impossibility of obtaining an arbitrary orientation of the vector of spontaneous polarization relative to khnicheskie result, which consists in making use of the obtained single crystals as pestujhely devices precise positioning, in particular a scanning probe microscopes, during alignment of optical systems.

This extension of the functionality of single crystals became possible due to the following:

- increase the area of domain boundaries and volume of domains;

- the possibility of orientation of the polarization vector of the domain at any given to domain wall angle;

- get domain structures as strictly regular in the whole volume, and any law they change (quasi-regulated, quasi-periodic).

This technical result is achieved as follows.

In the production method of a piezoelectric single crystals with polydomain structure for devices accurate positioning of the first form of the ferroelectric single crystals, in which formation is possible only 180data domain boundaries, the workpiece. In this workpiece, at least two faces parallel to each other. The perpendiculars to these parallel edges do not coincide with the direction of the axis of spontaneous polarization.

Then move the workpiece in thermal p is but to the parallel faces of the workpiece is applied periodically varying alternating electric field.

After cooling the entire volume of the workpiece below the Curie temperature it formed an ordered set of domain structure. The sizes of the domains in the structure are controlled by the speed of movement of the workpiece and the period of the polarity of the applied electric field thereto. Then divide the workpiece on the plate, two faces which are parallel to the domain boundary and contain an equal number of domains of opposite polarity.

In the particular case of the method for the orientation of the faces of the workpiece relative to the axis of spontaneous polarization is chosen from the condition of maximum values of the transverse piezoelectric effect in the piezoelectric single crystal.

As the ferroelectric single crystals using single crystals niobate or lithium tantalate.

In the method for producing thermal field use dual-zone tube furnace with a sharp temperature gradient in the region of the Curie temperature.

The thickness of domains of opposite polarity forming the same.

The moving speed of the workpiece is chosen equal to 0.5 cm/hour up to 5 cm/hour.

The use of the produced offered by way of single crystals as pestujhely devices accurate positivley a structure, in which two piezoelectric plates are interconnected so that the polarization vectors were oppositely directed.

Deflection Z of pleadies at his console consolidation can be represented by the expression

where h is the thickness of the element;

X0- its original length;

dmn- piezoelectric module;

U is the voltage on structure:

The coefficient of Electromechanical transmission element is determined by a ratio of

Pestujhely made of single-crystal plates with shaped domain structure can have coefficients of electro-mechanical transmission To not less than piezoceramic propulsion. This plate keeps monocrystalline and solidity, as the areas with different signs of polarization are not interconnected mechanically, so the border does not break the continuity of the crystal lattice, and the opposite signs of piezocomposites not lead to degradation of the inter-domain boundaries in time and the process repeated deformation during operation.

The conversion of pesedit cryogenic up to the Curie temperature.

The invention is illustrated in the drawing, in which Fig.1 presents an example implementation of the method of obtaining a polydomain structure in the single crystal of lithium niobate; Fig.2, 3, 4 - diagram of the formation of the blanks of ferroelectric single crystals of lithium niobate; Fig.5 - piezolighter in the device for the precise positioning is made of monocrystalline wafers based on the dual-domain lithium niobate, when bending down; Fig.6 - the same bending upward, and Fig.7, 8 - dependence of deformation obtained pestujhely of the voltage on the electrodes after annealing at different temperatures.

In Fig.1 shows a two-zone tube furnace 1, the electrodes 2, 3, cross-border, 4, 5, 6, vector 7 temperature gradient in the furnace, the vector 8 polarization domain, the angle 9 between the axis of spontaneous polarization of the crystal and the normal to the crystal face, which is applied to the electrodes, the crystal 10.

In Fig.5, 6 shows the directions 11, 12 deformation of domains with different directions of spontaneous polarization vectors.

A method of obtaining a piezoelectric single crystals with polydomain structure for precise positioning devices is as follows.

First form of segnetoelektrichestvo. As the ferroelectric single crystals can be used single crystals niobate or lithium tantalate.

The presence of spontaneous dipole moment determines the domain structure is unpolarized crystal ferroelectrics such as lithium niobate. Point group symmetry 3m with direction causes 180data of domains.

When the phase transition is offset along the polar Z-axis of the sublattices positive metal ions relative sublattice of oxygen atoms. The direction of displacement of cations determines the direction of the vector of spontaneous polarization [0001] magnetopause. An asymmetrical arrangement of metal ions in the structure of magnetopause causes the emergence of the dipole moment upon cooling crystals below the Curie temperature. There are two mutually opposite direction of the displacement of metal ions, corresponding 180th electric domains. For polarization reversal domains you want to transfer metal ions through oxygen layers that at temperatures close to room, impossible.

In the workpiece, at least two faces parallel to each other, which is necessary to create a homogeneous population to the axis of spontaneous polarization. The coincidence of these directions of deformation in all domains will have the same signs, which will lead to the absence of Flexural deformations in structures.

In the optimal case, the implementation of the method the orientation of the faces of the workpiece relative to the axis of spontaneous polarization is chosen from the condition of maximum values of the transverse piezoelectric effect in the piezoelectric single crystal.

Then move the workpiece in thermal field of the zone temperature is above the Curie temperature in an area with a temperature below the Curie temperature. Simultaneously to the parallel faces of the workpiece is applied periodically varying alternating electric field.

In the process of polarization of the layer of metal atoms is moved through a layer of oxygen atoms in the adjacent space between the atoms of oxygen, thereby changing the direction of orientation of the domain on the opposite.

This process can proceed at temperatures above the Curie temperature, at lower temperatures the position of the metal atom is fixed.

Ferroelectric single crystal, to which by means of electrodes applied periodically changing electric field undergoes a thermal zone with a temperature gradient, moving ITE electric field, which is applied to the crystal during cooling leads to the formation of domains, each pair of which the axis of spontaneous polarization antiparallel. The size of the domains is determined by the speed of movement of the workpiece in a heating zone and a switching period of the electric field.

The motion of the domain boundaries along the crystal stops when the magnitude of the external electric field becomes equal coercitive for a given composition of the crystal.

Repetition of the filing of the crystal electric field pulses of different polarity allows you to create a domain structure with any desired size of domains and the law changes over the crystal volume.

Optimal moving speed of the workpiece is chosen equal to 0.5 cm/hour up to 5 cm/hour. At speeds greater 5 cm/h thermal distortion of the front and significant violation of a flat shape of domain boundaries. At speeds less than 0.5 cm/hour each domain is in the alternating electric field for too long. It is possible prepolarization previously generated domains and, as a result, deterioration in the quality of domain boundaries.

In the Annex to the billet electric field intensity of less than 0.5 Smithie domain boundaries and the deterioration of structure in the polarization reversal of previously generated domains at temperatures below the Curie temperature.

As thermal unit is used a vertical tube furnace 1 (Fig.1) with resistive heating, in which is formed an axial temperature gradient vector 7 and a sharp temperature gradient in the region of the Curie temperature. The radial temperature gradient is eliminated by equalizing the temperature in the cross section.

After cooling the entire volume of the workpiece below the Curie temperature it formed an ordered domain structure with cross-border 4, 5, 6. Then divide the workpiece on the plate, two faces which are parallel to cross borders and contain an equal number of domains of opposite polarity. In particular, if the thickness of the domains of opposite polarity forming the same. In this case, when the supply of electric fields on the structure of the deformation of domains of different signs will be equal in magnitude and oppositely directed.

Example

To obtain a piezoelectric single crystals with polydomain structure for precise positioning devices carried electrothermal processing blanks cut from the ferroelectric single crystal 10 lithium niobate, congruent composition. Harvesting had the shape of a rectangular parallele is about focused on crystallographic directions X, Y+37and Z+37(Fig.2, 3, 4).

The orientation of the single crystal is determined by the need to obtain a piezoelectric domain structure with a maximum transverse piezoelectric effect when applying a field perpendicular to the domain wall.

On the verge Z+37blanks were caused by burn-conductive solid electrical current palladium electrodes 2, 3 with platinum leads to the filing of the crystal alternating electric field.

The billet was placed in a vertical dual-zone tube furnace 1 ohmic heating with an axial temperature gradient between zones 85S/cm so that the electrodes 2, 3 are arranged horizontally and perpendicular to the axial gradient in the furnace. In this case the angle between the vectors of the electric field and the axis of spontaneous polarization is 37. The upper hot zone of the furnace where the original is a billet is heated to a temperature of 1150With that 15Above the Curie temperature congruent composition of the single crystal. The lower cold zone of the furnace 1 is heated to a temperature of 900With that obespechyvalas volume which is heated above the Curie temperature, moves with a speed of 1.5 cm/hour from the hot zone to the cold zone. Ferroelectric phase transition and the formation of domains occurs between these zones.

Profile isotherms Curie temperature flat over the whole cross section of the crystal.

Simultaneously with the movement and cooling of the single crystal to the electrodes pulses alternating electric field with a strength of 0.5 In/see the Time of preparation under a field of one polarity is 120 seconds (the period of the polarity of the electric field is 240 seconds). The speed of movement of the workpiece and the time of switching the polarity of the field is selected from the conditions required size of the domains. At constant speed of movement of the workpiece in the furnace of 1.5 cm/h and the switching time of different pulse field after 120 seconds, the size of domains of different characters throughout the volume the same and equal to 500 μm, i.e., domains of opposite signs have the same thickness within the same period. Such a domain structure can be considered regular. The change ratio of the speed of switching and time switching field leads to a change in the size of the domains. When the selected geometry of the formation of domains of the vector of spontaneous polarization, always directed only along the polar the new domain structure carved plate, two faces which are parallel cross borders and contained two domains, the polarization vectors which were oppositely directed. Then using the grinding powders thickness patterns were brought to 1 mm, and the volumes of the domains are equal.

On the surface of the wafer, which is formed of piezoelectric single crystals with a polydomain structure, were applied to the electrodes to supply to them the control voltage, i.e., created bimorph structure, which can be used as pestujhely (Fig.5, 6).

The dependence of the deformation of the bimorph structures of the voltage on the electrodes when the console fixation was determined using a scanning tunneling microscope. In Fig.7 presents the results obtained for the original sample, Fig.8 presents the results obtained on the same sample, but annealed at a temperature of 800C for two hours. Deformation measurement was conducted at a distance of 10 mm from the point of fixation of the piezoelectric single crystal bimorph. The results show that the ratio of its Electromechanical transmission after annealing did not change, indicating a high structural stability. < polydomain structure for precise positioning devices, in which form of ferroelectric single crystals, in which formation is possible only 180data domain boundaries, the workpiece, in which at least two faces parallel to each other and perpendicular to these edges do not coincide with the direction of the axis of spontaneous polarization, then move the workpiece in thermal field of the zone temperature is above the Curie temperature in an area with a temperature below the Curie temperature with the simultaneous application of periodically varying alternating electric field to the parallel faces of the workpiece, after cooling the entire volume of the workpiece below the Curie temperature it formed an ordered domain structure, the sizes of the domains in which you can set the speed of movement of the workpiece and the period of the polarity of the applied electric field thereto, then divide the workpiece on the plate, two faces which are parallel to the domain boundary and contain an equal number of domains of opposite polarity.

2. The method according to p. 1, in which the orientation of the faces of the workpiece relative to the axis of spontaneous polarization is chosen from the condition of maximum values of the transverse piezoelectric effect in the piezoelectric shall monocristal niobate or lithium tantalate.

4. The method according to p. 1, wherein to generate thermal field use dual-zone tube furnace with a sharp temperature gradient in the region of the Curie temperature.

5. The method according to p. 1, wherein the thickness of domains of opposite polarity forming the same.

6. The method according to p. 1, wherein the moving speed of the workpiece choose equal to from 0.5 to 5 cm/h

7. The method according to p. 1, wherein the electric field is chosen not less than 0.5 and not more than 1.5 In/see

8. The method according to p. 1, wherein the period of the polarity of the electric field is 240 C.

 

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